1. Field of the Invention
This invention concerns the cutting of screw-threads. More particularly, this invention relates to a screw thread cutting apparatus and method for cutting screw threads on a surface, which surface may be either an interior surface (i.e., female screw threads) or exterior (i.e., male screw threads), and which may be a straight cylindrical surface or a tapering surface. Still more particularly, the present invention relates to such a screw thread cutting apparatus and method in which the starting end of the first screw thread is spaced away from an adjacent end of the threaded surface to provide a pilot portion or counter bore portion. Further, the starting end of the first screw thread is blunt or scalloped, so as to provide a jam-resistant start for the screw thread, and to avoid cross threading as well.
2. Brief Description of Related Technology
The use of a screw thread cutting tool including a milling cutter with a shank defining a recess in which is received a replaceable multi-tooth thread cutting insert is well known and conventional. These thread cutting tools are employed in computer numerically controlled (CNC) milling machines to cut both internal and external threads, which threads may be of straight (i.e., cylindrical) shape, or may be tapered (i.e., conical shape). The cutting of such screw threads on or within a work piece conventionally creates a thread form that runs right up to the end of an externally threaded part, or right up to the open end of a bore defined within an internally threaded part.
Conventional technology of the type described immediately above may be seen in U.S. Pat. Nos. 4,913,604; and 5,098,232
With the conventional technology, after the performance of an interior or exterior thread cutting operation, if it is desired to provide a starting pilot portion on a work piece having a male screw thread, or a starting counter bore portion within a work piece having a female threaded bore, then in each case the provision of such starting portions requires the performance of a second operation. That is, a second cutting operation must be performed using a tool to form the counter bore or pilot portion on the work piece. Further, when such a starting portion is provided on a male or female threaded part, the starting end of the first thread adjacent to the starting portion is most usually reduced to a thin edge, or to a fine-dimension. That is, at its starting end, the thread form tapers from a full thread shape to essentially a fine edge or toward a zero thickness. Such a fine starting end of a first thread also has a low thread form height, so that it is easily cross threaded in the event that threaded parts are not perfectly aligned when it is attempted to screw them together. That is, such a fine-dimension or thin starting end for a screw thread is fragile and is highly subject to damage. Such damage may cause jamming of threaded parts when assembly of these parts is attempted. Further, a fine-dimension or thin starting end on the first thread tapers to a low height which is easily crossed over by a matching thread and promotes cross threading of screw threaded parts if an attempt is made to assemble them with imperfect alignment. Such cross threading when it occurs can ruin the screw threads, and can possibly make the screw threaded parts unusable as well.